1. Technical Field
The present invention relates to a printed circuit board and particularly to a printed circuit board for transmitting high speed differential signals.
2. Description of Related Art
Flexible printed circuit boards (FPCs) are widely used in electronic devices for connecting different electronic elements of the electronic devices. The FPCs are made from flexible material, and have some advantages over conventional circuit boards. For example, the FPCs are very thin, and can be freely bent or folded to satisfy different layouts of the electronic elements of the electronic devices. Therefore, using FPCs to connect electronic elements can save space in the electronic device.
However, because FPCs are very thin, impedances of signal traces laid on the FPCs are very low, whereas high impedances will enable better performance in the transmission of high speed differential signals. For promoting the impedances of high speed differential signals, a conventional method is to etch a ground layer of the FPC into grids. In FIG. 5, a grid type ground layer 52 of an FPC 50 is shown. A pair of high speed differential signal traces 511 is laid on the FPC 50. When high speed differential signals are transmitted through the traces 511, common-mode noise is created, which affects the signal quality of the FPC.
Therefore, a new circuit board is desired to overcome the above-described deficiency.